I recently implemented OpenPilot for my vehicle and used it on a 9 hour road trip.

Due to the nuances of my vehicle, the built-in hands-on-wheel nag was still active and required (small) driver wheel inputs every 10 seconds to prevent the beeps. Just keeping both hands on the wheel was enough to pacify the nag. But this forced me to keep attention on the vehicle, more than OpenPilot's face direction detector. Anecdotally I am certain this kept me more engaged than if I could have gone hands off.

The net effect was I had more capacity to be aware of what was going on around me on the road, AND I didn't feel nearly as fatigued at the end of the drive. Keeping the car between the lines consumes a little bit of cognitive energy constantly, which compounds over a long drive. Freeing up that load allowed me to pay more attention to what was happening on the rest of the road. And the mental burden was still half as much--I felt like I had driven maybe 4-5 hours instead of 9.

I have some ideas for how I might disable the built-in nag, but I'm pretty sure doing so would make my car less safe. The current configuration might already achieve the safety maximum for ADAS based on front-facing cameras only.

It increases situational awareness while reducing fatigue, and I think that anything further I try to do will adversely affect one or both of those effects.